738 Part V / Movement
Figure 31–1 A typical muscle consists of many thousands
of muscle fibers working in parallel and organized into a
smaller number of motor units.A motor unit comprises a
motor neuron and the muscle fibers it innervates, illustrated
here by motor neuron A1. The motor neurons innervating one
muscle are usually clustered into an elongated motor nucleus
that may extend over one to four segments within the ventral
spinal cord. The axons from a motor nucleus exit the spinal
cord in several ventral roots and peripheral nerves but are
collected into one nerve bundle near the target muscle. In
the figure, motor nucleus A includes all those motor neurons
innervating muscle A; likewise, motor nucleus B includes all
the motor neurons that innervate muscle B. The extensively
branched dendrites of each motor neuron (not shown in the
figure) tend to intermingle with those of motor neurons from
other nuclei.
muscle. When the axon reaches the muscle, it branches
and innervates from a few to several thousand muscle
fibers (Figure 31–1).
Once synaptic input depolarizes the membrane
potential of a motor neuron above threshold, the neu-
ron generates an action potential that is propagated
along the axon to its terminals in the muscle. The
action potential releases acetylcholine at the neuro-
muscular synapse, triggering an action potential at the
sarcolemma of the muscle fiber (Chapter 12). A mus-
cle fiber has electrical properties similar to those of a
large-diameter, unmyelinated axon, and thus, action
腹根
运动核 B
运动
神经元
A1
运动核
A
肌肉 A
肌肉 B
肌肉
纤维
腱膜
腱
potentials propagate along the sarcolemma, although
more slowly due to the higher capacitance of the fiber
resulting from the transverse tubules (see Figure 31–9).
Because the action potentials in all the muscle fibers
of a motor unit occur at approximately the same time,
they contribute to extracellular currents that sum to
generate a field potential near the active muscle fibers.
Most muscle contractions involve the activation of
many motor units, whose currents sum to produce sig-
nals (compound action potentials) that can be detected by
electromyography. The electromyogram (EMG) is typi-
cally large and can be easily recorded with electrodes
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